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We report the analysis of 22 B-band light curves of the dwarf nova V4140 Sgrobtained with SOI/SOAR during two nights along the decline of a superoutburstin 2006 Sep 12-24 and in quiescence over 50 days following the superoutburst.Three-dimensional eclipse mapping of the outburst light curves indicates thatthe accretion disc is elliptical (eccentricity e=0.13) and that superhumpmaximum occurs when the mass donor star is aligned with the bulge of theelliptical disc. The accretion disc is geometrically thin both in outburst andin quiescence; it fills the primary Roche lobe in outburst and shrinks to abouthalf this size in quiescence. The stability of the eclipse shape, width anddepth along quiescence and the derived disc surface brightness distributionindicate that the quiescent accretion disc is in a high-viscosity,steady-state. Flickering mapping of the quiescent data reveal that thelow-frequency flickering arises from an azimuthally-extended stream-disc impactregion at disc rim and from the innermost disc region, whereas thehigh-frequency flickering originates in the accretion disc. Assuming thedisc-related flickering to be caused by fluctuations in the energy dissipationrate induced by magneto-hydrodynamic turbulence (Gertseema & Achterberg 1992),we find that the quiescent disc viscosity parameter is large (alpha ~ 0.2-0.4)at all radii. The high-viscosity quiescent disc and the inferred low disctemperatures in superoutburst are inconsistent with expectations of thedisc-instability model, and lead to the conclusion that the outbursts of V4140Sgr are powered by mass transfer bursts from its donor star.

SOAR observations of the high-viscosity accretion disc of the dwarf nova V4140 Sagitarii in quiescence and in outburst